There are two classes of pressure-sensitive adhesives, those in which the adhesive composition is a combination of an elastomer and a tackifier, i.e., a tackifying resin and those in which the adhesive composition is an acrylate in which both the adhesive strength and tack are provided by the acrylate polymer. The former is much preferred as an industrial pressure-sensitive adhesive in having superior adhesive characteristics.
In the manufacture of pressure-sensitive adhesive products in which the adhesive is of the class which employs a combination of elastomer plus tackifier, the adhesive components are intimately mixed, either in a solvent or under heat and pressure employing equipment such as a Banbury mixer, mill or extruder. The adhesive mixture then is applied onto a substrate or backing. The usual methods for accomplishing the application are solvent coating, calendering, hot melt coating, and extruding.
Solvent coating is undesirable for several reasons. It may be expensive and/or wasteful. Formerly, the spent solvent was exhausted to the atmosphere. This was both wasteful and pollution producing. To avoid pollution, afterburners or solvent recovery systems are employed. The use of afterburners does not avoid waste and the use of solvent recovery systems is quite expensive.
The solvent-free methods, i.e., calendering, hot-melt coating, extruding, usually require one or more heavy duty equipment which is expensive to install and to operate. The calendering method, in addition, is not adaptable for preparing thin adhesive layers.
A coating method known in the non-adhesive art is the application of powder to a substrate surface, heating to fuse the particles to a molten condition and cooling to obtain a film coating. This has been carried out successfully with a single-component non-adhesive powder such as polyethylene. This procedure would be desirable for preparing pressure-sensitive adhesive coatings because it is adaptable to conventional ovens, is less costly, and is applicable in either light or heavy weight coatings. Although non-pressure-sensitive adhesives have been applied to substrate as powders, these have been hot melt adhesives in which bonding is carried out while the adhesive is in the fused state. These procedures do not require formation of a smooth film coating on cooling. Smooth pressure-sensitive adhesive coatings have not heretofore been found to be obtainable from a mixture of elastomer and tackifier employing the powder method. While U.S. Pat. No. 3,573,128 describes an apparatus and method for pressure-sensitive adhesive tapes, there is no teaching or example indicating the chemical nature of the material or materials which may be employed.
It is an object of the present invention to find a suitable means for successfully adapting the powder film forming method for the preparation of pressure-sensitive adhesive tapes.